System for detecting the level of liquid in a tank

ABSTRACT

The detecting system comprises a detecting circuit ( 1 ) made up of a transistor (T 1 ) in current generator configuration, connected in series with a resistance (Rs) and a capacitor ( 13 ) of known capacitance (Cx), the resistance Rs representing the resistivity of the liquid existing between two electrodes ( 5, 6 ) plunged into the liquid. Thanks to the presence of the capacitor ( 13 ), it is possible to detect the liquid level in relation to the integral of the current that charges the capacitor ( 13 ), resistance Rs being such as to condition the charge time (Tp) of the capacitor ( 13 ). Accordingly, the variations in the voltage drop (Vp) on Rs caused by successive charges of the capacitor ( 13 ) with a current of time Tp represent corresponding values of the level of the liquid.

This is a U.S. National Phase Application Under 35 USC 371 and applicantherewith claims the benefit of priority of PCT/IT2003/000427 filed onJul. 8, 2003, which was published Under PCT Article 21(2) in English,and of Application No. TO2002A000601 filed in Italy on Jul. 10, 2002.The contents of the applications are incorporated by reference herein.

TECHNICAL FIELD

This invention relates to a system for detecting the level of a liquidin a tank, and more particularly the level of ink contained in thecartridge of an ink jet printhead.

BRIEF DESCRIPTION OF THE STATE OF THE ART

Various devices and relative methods are known in the sector art, and inparticular in the field of ink jet printers, for detecting the residualink quantity, or the level of ink in the cartridge of the printhead, themost widely used of which may be divided into the following categories:

devices for detecting the residual quantity of ink by means of floatsystems;

detecting devices that use the measurement of a rapid variation of theelectrical resistance of the ink between two electrodes, when one ofthese remains uncovered, of the ON-OFF type;

finally, detecting devices that use the measurement of the variation ofthe resistance of the ink between two electrodes placed in a porous bodyimpregnated with ink.

These devices have drawbacks of different types, such as lack ofprecision, mechanical failures and jamming for all the float detectortypes, lack of precision and incorrect measurement of the resistance ofthe ink for all the other types of detectors, mainly on account of thefact that the resistance is highly dependant on the temperature and onthe composition of the ink.

What is more, the residual ink quantity detecting devices known in thesector art are unsuitable for working in the presence of electricallynon-conducting liquids, such as for instance oily liquids and liquidsderived from crude oil.

SUMMARY DESCRIPTION OF THE INVENTION

One object of this invention, therefore, is to overcome the drawbacksfound in the type of detectors known in the art and classified above.

Another object of the invention is to produce a system for detecting thelevel of a liquid in a tank in such a way that the indication of thelevel of the liquid is independent of the influence of parameters, suchas temperature, composition of the liquid, in particular an ink, and anyerrors in the dimensions and position of the detecting electrodes.

Finally a further object of this invention is to produce a system fordetecting the level, or quantity of liquid in a tank, filled with anelectrically non-conducting liquid.

Therefore in accordance with the stated objects of this invention, asystem is proposed for detecting the level of a liquid in a tank, thedetecting system comprising:

at least two electrodes (5, 6) extending into the inside of said tank(2), in contact with said liquid, said electrodes (5, 6) being separatedby a volume of liquid (8) presenting an own electrical resistance (Rs)variable in function of the level of the liquid in said tank, andinfluenced by environmental conditions and by physical properties ofsaid liquid,

detecting means (1) electrically connected to said electrodes (5, 6),and

a control unit (CPU) suitable for controlling said detecting means,characterized as defined in the main claim.

These and other characteristics of the invention will appear moreclearly from the following description of a preferred embodiment,provided by way of non-restrictive example, with reference to thefigures of the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 represents a wiring diagram of the detecting system according tothe invention;

FIG. 2 depicts schematically one of the arrangements of the electrodesinside the ink cartridge;

FIG. 3 shows a diagram of the standard variation of resistivity of theink with ambient temperature;

FIG. 4 represents a different embodiment of the detecting system of FIG.1;

FIG. 5 represents a version of the device of FIG. 1 adapted fordielectric liquids;

FIG. 6 represents schematically the disposition of a pair of capacitiveelectrodes for dielectric liquids;

FIG. 7 shows a variant of the diagram of FIG. 5.

DETAILED DESCRIPTION OF THE INVENTION

Although the discovery according to this invention may be used formeasuring the level of any liquid contained in a tank, the descriptionthat follows focuses, for simplicity's sake, on an application,certainly non-restrictive, of the system for detecting the level of aliquid used for measuring the ink level of a cartridge 2 (FIG. 2) of anink jet printhead 3, integral with the cartridge 2.

As a non-restrictive, illustrative example of the above-mentionedapplication, reference is made to use of the printhead 3 on slipprinting equipment, used at points of sale (POS) in shopping centres,such as that described in the Italian patent application No. TO 2002000428, filed by the Applicant, which may be consulted for greaterdetails about using the system for detecting the level of a liquidaccording to this invention.

The ink jet printhead 3 is of a type widely known in the sector art, andis therefore not described in detail, the reader being referred insteadto Italian patent no. 1.233.061, for example.

The cartridge 2 contains a spongy body 4 of electrically insulatingmaterial and provided with high porosity which, when the cartridge isnew, is completely impregnated with ink, which is held on the inside bycapillarity.

Two metallic electrodes 5 and 6 are attached to a lid 7 of the cartridge2, located at the opposite end with respect to the printhead 3, andextend inside the spongy body 4; the electrodes 5 and 6 are thereforemaintained in contact with the ink contained in the cartridge 2.

The two electrodes 5 and 6 are arranged at a distance from one another,functionally pre-established as to define between them a volume 8 ofink, which presents a determined electrical resistance Rs to the passageof an electrical current of constant and defined intensity.

Gradually as the ink is consumed by the head 3 for printing, the spongybody 4 empties, the ink inside the spongy body 4 recedes starting fromthe lid 7, in the direction of the printhead 3. The volume 8 of inkbetween the electrodes 5 and 6 diminishes, and as a result theresistance Rs increases in value.

In practice, however, the resistance Rs is influenced by other parasiticparameters that modify its value, causing errors during the detection ofthe level of ink in the cartridge 2, if they are not taken into dueconsideration. These parasitic parameters are first and foremost theenvironmental conditions that the cartridge 2 is in at the time thelevel of ink is measured, and the chemical composition of the ink. Theenvironmental condition that most influences the resistance Rs istemperature. FIG. 3 shows a typical trend of the variation of the valueof the resistance Rs, or resistivity, upon variation of the ambienttemperature. It may be noted that, for the most commonly used types ofink in ink jet printers, in the ambient temperature range from 10° C. to30° C., resistivity of the ink varies on average from 1.6 to 0.8approximately, once the resistivity at 20° C. has been fixed at 1.

The main object of this invention, therefore, is to be able to detectthe level of the ink in the cartridge 2, regardless of the influence ofthe parasitic parameters mentioned above.

With reference to FIG. 1, the numeral 1 is used to indicate as a whole adetecting circuit, comprising the detecting means according to theinvention. The circuit 1 comprises a transistor T1, configured as acurrent generator, polarized by means of two fixed resistances Ra andRb, fed with power by a voltage source +V; the point 9 of commonconnection of the resistances Ra and Rb is connected to the base 10 oftransistor T1.

The emitter 11 of T1 is connected to one end B of the resistance Rs,i.e. it is connected to the corresponding electrode 6 (FIG. 2) of thecartridge 2, whereas the other end A of the resistance Rs, correspondingin turn to the electrode 5, is directly connected to the source +V.

In order to render measurement of the level of ink independent of theinfluence of the parasitic parameters mentioned earlier, according tothe invention a capacitor 13, of known and constant capacitance Cx, hasbeen included in the circuit of FIG. 1. The capacitor 13 is connectedbetween the collector 12 of T1 and the common earth M.

An FET transistor T2 is connected in parallel to the ends of thecapacitor 13, its function being to discharge the capacitor 13, aftereach measurement, in order to re-establish its starting conditions for anew measurement. For this purpose, the transistor T2, normally inactive,is activated by a pulse applied in a known way to the gate 15.

A third transistor T3 is placed in series with the resistances Ra, Rb,between the resistance Rb and the earth M. The transistor T3 is normallyinactive, but is activated by means of a signal S generated by the CPU,of predefined duration, as will be described later, and applied to thebase 14 of the transistor T3, only when it is necessary to make thetransistor T1 conducting, i.e. when the level of ink has to be measured.

Inclusion of the capacitor 13 permits generation of a signalrepresenting the level of liquid, namely of ink in the cartridge 2, nolonger proportional to the current flowing through a referenceresistance in series with the resistance offered by the ink, as wasknown in the sector art, for example as in a device described in U.S.Pat. No. 5,162,817, but proportional to the integral of the current.This leads to a different form of behaviour of the circuit 1, when it ispowered with a constant current I.

In fact, if we indicate with Vp the voltage drop measured on theterminals A, B of the resistance Rs, with Vx the voltage on theterminals of the capacitor 13 of capacitance Cx, we obtain the relation1):Vx=(TpVp)/(CxRs)  1)

in which Tp indicates the duration of the time to charge the capacitor13 up to a voltage Vx, i.e. the driving time of the circuit 1. Howeverthe voltage drop Vp, being proportional to the resistance Rs, will alsosuffer the variations due to the above-mentioned parasitic parameters,and will not give a true indication if used as a quantity representingthe level of ink.

If the relation 1) is interpreted in another way, where the voltage Vxis considered to be constant, the following relation 2), derived from1), is obtained:Vp=VxCxRs/Tp,  2)

in which the driving time Tp is now the quantity proportional to Rs.

In other words, by using a certain cartridge 2, and by driving thecircuit 1 with a current I until a prefixed voltage Vx is obtained,depending for instance on the dynamics of the A/D conversion equipment29, a driving time Tp1 is obtained, that can easily be measured by meansof a digital counter 30 and stored in a memory, for instance in a memory16 (FIG. 1, 2) located on board the cartridge itself.

Afterwards, as the ink in the cartridge 2 is gradually used forprinting, by subsequently driving the circuit 1 with a pulse of currentI of duration equal to the time Tp1, corresponding values of the voltagedrop Vx may be measured, which will give a true representation of thepattern of the level of the ink as they are no longer affected by theinfluences of the parasitic parameters, now automatically compensatedfor by the value of Tp1. Therefore in this mode of operation of thecircuit 1, just described, the voltage drop on the terminals of thecapacitor 13 is truly representative of the level of ink contained inthe cartridge 2.

Method of Measuring the Level of Liquid

The method of measuring the level of a liquid contained in a tank, andin particular the level of ink in a cartridge 2 of an ink jet printhead3, is conducted according to the following steps:

step 1): each newly produced cartridge, filled with a certain type ofink, black or colour, is connected to the circuit 1, which is poweredwith a current I;

step 2): the ambient temperature surrounding the new cartridge ismeasured;

step 3): the counter 30 is used to measure the driving time Tp used toreach a voltage Vx that is fixed in advance and suitable for thecharacteristics of the converting/measuring device 29 used fordigitizing and measuring the voltages;

step 4): the time Tp measured in step 3) is associated with a standardambient temperature of 20° and is stored in the non-volatile memory 16fixed on board the cartridge 2; this measured time Tp represents themaximum value of the level of ink contained in the new cartridge 2;

step 5): in use with the cartridge 2, mounted on printing equipment andconnected to the detecting circuit 1, still controlled by the CPU, theambient temperature of the cartridge during the operating step ismeasured;

step 6): the detecting circuit 1 is powered with a pulse of current I ofduration equal to the driving time Tp, taken from the memory 16, and thevoltage drop Vx on the terminals of the capacitor 13 is measured, beforebeing converted by the converter 29, connected to the control unit orCPU;

step 7): the measurement of Vx is associated with the temperature of 20degree., in such a way as to obtain the corresponding value of the levelof ink, converted according to a suitable scale;

step 8): the measurement is repeated as in steps 6) and 7), each timethe value of level of the ink in the cartridge 2 needs to be known;

step 9): the cartridge 2 is replaced when a voltage drop Vx is detectedon the capacitor 13 equal to or less than a previously calculated limitvalue, indicative of a situation of cartridge empty.

FIG. 3 shows the influence of the ambient temperature on the resistanceof various inks; the diagram of FIG. 3 is drawn in dimensionless andnormalized form, in which the value 1 on the ordinates corresponds tothe resistance of the ink at 20° C.

FIG. 4 represents a different embodiment 17 of the detecting circuit 1of FIG. 1, suitable for use with digitizing and measuring equipmenthaving a floating input, i.e. not referred to the earth; in the circuit17 the resistance Rs and the capacitor 13 are changed over and thetransistor T3 is connected between the +V pole and the resistance Ra; atransistor T4 has been added together with the charging 18 andpolarizing 19 resistance of T3; this addition does not affect globaloperation of the circuit 17.

FIG. 5 shows a detecting circuit 20 equivalent to that of FIG. 1, andwhich therefore works in the same way, suitable for non-conducting, ordielectric, liquids, such as for example liquids used in the chemicalindustry, liquids derived from crude oil, mineral oils, or vegetableoils, etc.

The capacitor 22 represents the capacitance Cx of a pair of electrodes23 and 24 (FIG. 6), facing one another and immersed in the liquid 25,the level of which in a tank 26 has to be measured. The capacitance Cxvaries depending on the level of the liquid in the tank 26, as a portionof the electrodes remains uncovered and therefore the dielectricconstant which defines the capacitance Cx varies as a result.

Finally the circuit 27 of FIG. 7 is a detecting circuit equivalent tothat of FIG. 5, in which the resistance Re and the capacitor 22 arechanged over, should the voltage drop Ve be measured by floating inputequipment, without earth reference.

It remains understood that changes, additions or component partsubstitutions may be made to the system for detecting the level of aliquid in a tank, according to this invention, without departing fromthe scope of this invention.

1. System for detecting the level of a liquid in a tank, comprising: at least two electrodes extending into the inside of said tank, in contact with said liquid, said electrodes being separated by a volume of liquid presenting an own electrical resistance variable in relation to the level of the liquid in said tank, wherein said variable resistance is influenced by environmental conditions and by physical properties of said liquid; detecting means electrically connected to said electrodes, and powered by a voltage source; and a control unit suitable for controlling said detecting means; wherein said detecting means comprise a capacitance connected in series to said resistance, and a current generator connected in series between said resistance and said capacitance, said current generator being suitable for being activated by said control unit for powering said resistance and for charging said capacitance with said current until a predefined voltage is reached on the terminals of said capacitance, during a corresponding charge time representative of the current level of said liquid in said tank, and wherein said control unit is prearranged for storing said charge time in a memory and for activating at later times said generator for a duration equal to said stored charge time, so that said capacitance is charged with a current such as to produce on said capacitance a voltage drop proportional to the variation of said resistance caused by a corresponding variation of the level of said liquid, and independent of said physical properties.
 2. Detecting system as in claim 1, wherein the value of said voltage drop equal to or less than said predefined value represent with continuity corresponding levels of said liquid equal to or less than said maximum level.
 3. Detecting system as in claim 1, wherein said physical properties comprise the chemical composition of said liquid.
 4. Detecting system as in claim 1, wherein said tank consists of a cartridge for the ink of an ink jet printhead, said cartridge being filled with a porous body impregnated with said ink.
 5. Detecting system as in claim 4, wherein said electrodes are inserted in said porous body, and that said electrical resistance corresponds to the resistance of a volume of said ink between said electrodes, and variable in relation to the consumption of ink by said printhead.
 6. Detecting system as in claim 1, wherein said detecting means comprise a non-volatile memory suitable for storing said charge time representative of the maximum level of said liquid in said tank.
 7. Detecting system according to claim 6, wherein said non-volatile memory is integral with said cartridge.
 8. Detecting system as in claim 1, wherein said detecting means comprise a first transistor connected in series between said resistance and said capacitor and selectively polarized by a pair of fixed resistances, said detecting means being powered by said voltage source, and a second transistor, wherein said second transistor is normally off and is connected in series to said pair of resistances, said second transistor being activated by a signal of duration equal to said representative charge time, so that said first transistor is actuated for charging said capacitor with a current, such as to produce on said capacitance a voltage drop representative of the level of said liquid.
 9. Method for detecting the level of a liquid in a tank, comprising the following steps: providing at least two electrodes extending into the inside of said tank, in contact with said liquid, whereby said electrodes are separated by a volume of liquid presenting an own electrical resistance variable in relation to the level of the liquid in said tank; providing detecting means electrically connected to said electrodes, and powered by a voltage source, said detecting means comprising a capacitance connected in series to said resistance, and a current generator connected in series between said resistance and said capacitance; activating said current generator for powering said resistance and for charging said capacitance with a first current until a predefined voltage is reached on the terminals of said capacitance; measuring a corresponding charge time of said capacitance as representative of the current level of said liquid in said tank; storing said charge time in a memory; activating at later times aid current generator for a duration equal to said stored charge time, whereby said capacitance is charged with a second current such as to produce on the terminals of said capacitance a corresponding voltage drop; and measuring said voltage drop as suitable for obtaining from it the level of said liquid in said tank. 